Plasma display device driven in a subframe mode

ABSTRACT

The plasma display device has a plasma display panel and a driving part for driving the plasma display panel in a subframe mode. The driving part has a circuit for calculating the length of one frame based on one period of a vertical synchronizing signal introduced along with an image signal from an external device, a circuit for calculating the total number of sustaining pulses contained in one frame based on a brightness information contained in the image signal, and a circuit for calculating the length of one driving period required for displaying one frame. The length of one frame and the length of one driving period thus obtained are then compared in a comparing circuit. If the one frame length is found to be shorter than the one driving length, the total number of sustaining pulses or the number of scan lines will be reduced so that the one frame length becomes shorter than the length of one driving period, thus avoiding an extraordinary display.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma display device, and inparticular, it relates to a plasma display device driven in a subframemode.

2. Description of the Related Art

A plasma display panel (referred to as PDP, below) is a kind of flatdisplay widely used, for example, in various OA apparatus and TVs, sincethe panel structure thereof is very simple and all the elements of thepanel structure can be made using a thick film printing technique.

A conventional color PDP of a triple-electrode type is structured withtwo glass plates arranged in parallel with each other to form adischarge space. On one of the glass plates, address electrodes and aphosphor are provided while X electrodes and Y electrodes are providedon another glass plate to intersect each other at right angles. Aso-called "subframe mode" is known to drive such a PDP of thetriple-electrode type. In this driving mode, one frame is divided into,for example, 8 subframes, each of which has a sustaining dischargeperiod. The respective sustaining discharge periods of the subframes areset to a ratio of 1:2:4:16:32:64:128 (although the ratio is constant inthis example, there is no need for it to be always constant), and thesesubframes are combined to realize a grey-scale display.

In such a plasma display device driven in the subframe mode, thebrightness of the PDP is determined by the total number of sustainingpulses applied to the PDP during one frame. That is, it is determined bythe total number of sustaining pulses in all the subframes of one frame.In actuality, as the number of sustaining pulses applied to electrodesduring one frame increases, the brightness of the display increases.Therefore, to display a bright image on the plasma display device, alarge number of sustaining pulses are necessary during one frame while asmall number of sustaining pulses are enough to display an image havinga relatively low brightness.

PDPs are usually driven by image signals supplied from an externaldevice, such as a TV tuner and a personal computer, connected to thePDPs. The driving frequencies of these external devices are not the sameas each other. Since the length of one frame of a PDP is determineddepending on one period of a driving signal, that is, a verticalsynchronizing signal, introduced from an external device, the reallength of one frame of the PDP varies depending on what kind of externaldevice is connected with the plasma display device.

According to the variation of the frame length as mentioned above, adeficiency arises as follows. When the frame length becomes shorter thanthat expected for the plasma display device, the length of one drivingperiod of the plasma display device required to display one frameexceeds the real frame length when displaying a very bright image with alarge number of sustaining pulses. This results in an extraordinarydisplay on the plasma display device.

On the other hand, when the frame length becomes much longer than thatexpected in advance for the plasma display device, the length of onedriving period of the plasma display device required to display oneframe becomes shorter than the expected value. This results in theunnecessary extension of a quiescent period in one frame, thus loweringthe brightness of the display.

As mentioned above, the prior art plasma display device driven in asubframe mode is disadvantageous in that it does not have enoughflexibility to accommodate various types of external devices havingdifferent driving frequencies.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above mentioneddisadvantage of the prior art plasma display device. Therefore, anobject of the present invention is to provide a plasma display devicedriven in a subframe mode, the device having enough flexibility toaccommodate various types of external devices having different drivingfrequencies.

Another object of the present invention is to provide a plasma displaydevice which is capable of reducing the total number of sustainingpulses to avoid an extraordinary display when the length of one frame isshorter than the length of one driving period required to display oneframe.

Still another object of the present invention is to provide a plasmadisplay device which is capable of increasing the total number ofsustaining pulses to increase brightness of the display when the lengthof one frame is longer than the length of one driving period required todisplay one frame.

Still another object of the present invention is to provide a plasmadisplay device, which is capable of adjusting the length of the drivingperiod in a manner to shorten the length of one frame by reducing orincreasing the number of scan lines of the plasma display device.

In order to realize the above mentioned objects, the plasma displaydevice according to one feature of the present invention has a plasmadisplay panel and a driving part for driving the plasma display panel ina subframe mode. The driving part further has a circuit for calculatingthe length of one frame for display according to one period of avertical synchronizing signal introduced with an image signal from anexternal device, a circuit for detecting the total number of sustainingpulses contained in one frame according to brightness informationcontained in the image signal and a circuit for calculating one drivingperiod of the plasma display device required to display one frameaccording to the total number of sustaining pulses thus obtained. Thecalculated length of one frame and that of one driving period arecompared each other by a comparing circuit. The driving part further hasa circuit for changing the total number of sustaining pulses containedin one frame according to the compared result from the comparingcircuit. In a case where the calculated length of one frame is shorterthan that of one driving period, the changing circuit reduces the totalnumber of sustaining pulses to avoid an extraordinary display. On thecontrary, when the calculated length of one driving period is shorterthan that of one frame length, the changing circuit increases the totalnumber of sustaining pulses to increase the brightness of the panel.

A circuit is further provided for finding the lapse of a constant timesince the comparing circuit found a change in the comparing result. Thechanging circuit changes the total number of sustaining pulses containedin one frame after the time lapse finding circuit detects the lapse ofthe constant time.

The frame length calculating circuit, the driving period calculatingcircuit and the comparing circuit may be made by a microprocessor unitand a media in which a program is stored to operate the microprocessorunit as these circuits.

In another feature of the present invention, the plasma display devicehas a plasma display panel and a driving part for driving the plasmadisplay panel in a subframe mode. The driving part further has a ROMtable having a plurality of addresses, in each of which a combination ofthe number of sustaining pulses in the respective subframes is stored, acircuit for calculating the length of one frame for display according toone period of a vertical synchronizing signal introduced with an imagesignal from an external device, a circuit for detecting the total numberof sustaining pulses contained in one frame based on an address of theROM table, the address which corresponds to a brightness informationcontained in the image signal, and a circuit for calculating the lengthof one driving period of the plasma display panel necessary fordisplaying one frame based on the total number of sustaining pulses thusdetected. The calculated length of one frame and that of one drivingperiod are compared each other by a comparing circuit. The drivingcircuit further has a circuit for changing the address of the ROM tableaccording to the compared result from the comparing circuit. In a casewhere the calculated length of one frame is shorter than that of onedriving period, the changing circuit changes the address of the ROMtable in a manner to reduce the total number of sustaining plusescontained in the address so that the length of the driving periodbecomes shorter than the length of one frame. On the contrary, when thecalculated length of one driving period is shorter than that of oneframe length, the changing circuit changes the address of the ROM tablein a manner to increase the total number of sustaining pluses containedin the address so that enough brightness can be obtained.

A circuit is further provided for finding the lapse of a constant timesince the comparing circuit found a change in the comparing result. Thechanging circuit changes the address of the ROM table after the timelapse finding circuit detects the lapse of the constant time.

The frame length calculating circuit, the driving period calculatingcircuit and the comparing circuit may be made by a microprocessor unitand a media in which a program is stored to operate the microprocessorunit as these circuits.

In still another feature of the present invention, the plasma displaydevice of the present invention has a plasma display panel having aplurality of light emitting cells arranged in form of a matrix and adriving part for driving the plasma display panel in a subframe modewhile scanning the plurality of light emitting cells line-sequentially.The driving part further has a circuit for calculating the length of oneframe for display according to one period of a vertical synchronizingsignal introduced with an image signal from an external device, acircuit for detecting the total number of sustaining pulses contained inone frame according to brightness information contained in the imagesignal and a circuit for calculating one driving period of the plasmadisplay panel required to display one frame according to the totalnumber of sustaining pulses thus obtained. The calculated length of oneframe and that of one driving period are compared each other by acomparing circuit. The driving part further has a circuit for changingthe total number of scan lines, which are scanned in line-sequence,according to the result from the comparing circuit. In a case where thecalculated length of one frame is shorter than that of one drivingperiod, the changing circuit reduces the total number of scan lines toshorten the addressing periods uniformly in one frame, and so the lengthof one driving period, to avoid an extraordinary display. On thecontrary, when the calculated length of one driving period is shorterthan that of one frame length, the changing circuit increases the totalnumber of scan lines to enlarge a display range.

A circuit is further provided for finding the lapse of a constant timesince the comparing circuit found a change in the comparing result. Thechanging circuit changes the total number of scan lines after the timelapse finding circuit detects the lapse of the constant time.

The frame length calculating circuit, the driving period calculatingcircuit and the comparing circuit may be made by a microprocessor unitand a media in which a program is stored to operate the microprocessorunit as these circuits.

In still another feature of the present invention, the plasma displaydevice has a plasma display panel and a driving part for driving theplasma display panel in a subframe mode. The driving part further has acircuit for calculating a length of one frame for display according toone period of a vertical synchronizing signal which is introduced froman external device along with an image signal, a circuit for calculatinga maximum number of sustaining pulses, which can be driven withoutcausing any extraordinary display, based on the length of one frame thusobtained and for calculating the maximum brightness according to themaximum number of sustaining pulses, and a circuit for comparing themaximum brightness with a predetermined referential brightness. Acircuit is further provided for calculating a number of sustainingpulses which corresponds to the predetermined referential brightnesswhen the maximum brightness is found to be different from thepredetermined referential brightness, and for setting the calculatednumber of sustaining pluses to be the maximum number of sustainingpluses. As a result, a constant brightness can be obtained without beingaffected by the variation of driving frequencies of external devices.

The present invention further provides a driving method for the plasmadisplay panel. In this method, the length of one frame for display iscalculated according to one period of a vertical synchronizing signalwhich is input from an external device along with an image signal. Atthe same time, the total number of sustaining pulses contained in oneframe is detected based on brightness information contained in the imagesignal. Then, the length of one driving period of the plasma displaypanel necessary for displaying one frame is calculated based on thetotal number of sustaining pulses detected in the previous step.Thereafter, the calculated frame length and the length of one drivingperiod are compared, and the compared result is used to change the totalnumber of sustaining pulses. In a case where the frame length is shorterthan the length of one driving period, the total number of sustainingpulses will be reduced so that the one frame length becomes shorter thanthe length of one driving period. As a result, an extraordinary displaycaused by the driving period longer than the frame length caneffectively be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein:

FIG. 1 is a view for explaining a subframe mode for driving a PDP;

FIGS. 2(a) to 2(c) are views showing examples of waveforms applied onaddress electrodes, X electrodes, and Y electrodes of the PDP;

FIG. 2(d) is a view showing the definition of a reset period, addressingperiod and sustaining discharge period in one frame;

FIG. 3 is a view showing an example of stored data in a ROM table forcontrolling the brightness of the PDP;

FIG. 4 is a view for explaining a principle structure of the presentinvention;

FIG. 5(a) is a view showing the entire structure of a plasma displaydevice according to one embodiment of the present invention;

FIG. 5(b) is a view showing the detailed structure of the common drivercontrol part shown in FIG. 5(a);

FIG. 6 is a view showing the detailed structure of a part of the plasmadisplay device shown in FIGS. 5(a) and 5b;

FIG. 7 is a view showing an example of stored data in a ROM table whichis used for explaining embodiments of the present invention;

FIG. 8 is a view showing a flowchart according to the first embodimentof the present invention;

FIG. 9 is a view showing a flowchart according to the second embodimentof the present invention;

FIG. 10 is a view showing a flowchart according to the third embodimentof the present invention;

FIG. 11 is a view showing a flowchart according to the fourth embodimentof the present invention;

FIG. 12 is a view showing a flowchart according to the fifth embodimentof the present invention;

FIG. 13 is a view showing a flowchart according to the sixth embodimentof the present invention;

FIG. 14(a) is a view showing a flowchart according to the seventhembodiment of the present invention; and

FIG. 14(b) is a view showing the detailed structure of the common drivercontrol part shown in FIG. 5(a) for realizing the seventh embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the embodiments of the present invention, the relatedart and the disadvantages thereof will be described with reference tothe related figures.

FIG. 1 is a schematic view for explaining the frame structure of thesubframe mode. In this example, one frame is divided into 8 subframesSF1 to SF8, each subframe having three kinds of period, that is, a resetperiod, an addressing period and a sustaining discharge period. Eachlength of the first two periods is constant in every subframe while thesustaining discharge periods t1 to t8 are different with a constantratio in each subframe. In FIG. 1, L1, L2, . . . and LN denotehorizontal scan lines and the diagonals in the address period of eachsubframe imply that horizontal scan lines L1, L2, . . . and LN areselectively driven line-sequentially.

A conventional driving method driven in the subframe mode will beexplained next with reference to the waveform diagrams shown in FIGS. 2ato 2d.

FIG. 2a is a timing chart of a waveform applied to address lines withinone subframe, FIG. 2b is that applied to X electrodes, and FIG. 2c isthat applied to Y electrodes. In addition, FIG. 2d defines the resetperiod, the addressing period, and the sustaining discharge period inthe respective waveform charts. In the explanation below, voltages areindicated by way of example and therefore, the method is not restrictedto the voltage values described.

In the reset period, all the Y electrodes are set to a 0 V level atfirst. At the same time, in order to form enough potential, afull-screen writing pulse of about +330 V level is applied to all the Xelectrodes while maintaining all the address electrodes at about the+110 V level. As a result, a discharge takes place in all the cellsregardless of the previous display states of these cells.

Thereafter, the address electrodes and the X electrodes are set to0-level to cause a discharge in all the cells. In this case, thepotentials across the electrodes are kept 0 V level so that thedischarge ceases as a result of self saturation without forming any wallcharges. This discharge is called a self-erasure discharge. As a resultof this self-erasure discharge, all the cells in the panel are reset toa uniform state having no wall charge. This reset period is provided toset all the cells into the same state regardless of the lighting statesin the previous subframe and to stabilize the address (write) dischargein the next address period.

In this reset period, a step may be provided to apply the firstauxiliary pulse Vass1, the second auxiliary pulse Vass2 and theauxiliary erasure pulse Vae on the Y electrodes, in order to eliminatingthe wall charges on the Y electrodes. In this case, pulses of +110 Vlevel are applied to the address electrodes during the application ofthe auxiliary pulses.

During the address period, the panel is scanned line-sequentially inorder to turn on or off the cells according to display data, thusconducting address discharge. First, the Y electrodes are scannedline-sequentially with pulses (referred to as scan pulses, below) ofabout -150 to -160 V while keeping the voltage of the X electrodes about+50 V. At the same time, pulses of about +60 V (referred to as addresspulses, below) are selectively applied to address electrodes, whichcorrespond to cells to cause sustaining discharge, that is, to causeselective light emission. As a result, an electric potential of about210 to 220 V, which is enough to cause discharge, is generated acrossthe address electrodes to which the address pulses are applied and the Yelectrodes to which the scan pulses are applied, thus causingaddress-discharges across these electrodes. On the other hand, since theelectric potential across the X electrodes and the Y electrodes, onwhich the scan pulses are applied, are about 200 V to 210 V, which isabout 10 V less than that of across the address electrodes and the Yelectrodes, no self-discharge takes place across them. However,discharge takes place between the X electrodes and the Y electrodesusing the address-discharges as a trigger, thus forming wall charges onparts of the dielectric layer corresponding to the cross points of the Xand Y electrodes.

During the sustaining discharge period (referred to as a sustainingperiod, below), pulses of about +180 V (referred to as sustainingpulses, below) are applied to the X electrodes and the Y electrodesalternately. As a result, sustaining discharges take place between the Xand Y electrodes where the wall charges have been accumulated during theprevious address period, thus displaying an image of one subframe on thePDP. In this situation, a voltage of about 110 V is applied to theaddress electrodes in order to avoid discharges across the addresselectrodes and the X electrodes.

In the above mentioned driving method, called a "addressing/sustainingdischarge separated writing addressing method", the brightness of thepanel depends on the length of the sustaining period, that is, thenumber of sustaining pluses applied during this period. Since the periodof one sustaining pulse does not change throughout all the subframes,the number of sustaining pulses in the respective subframes shown inFIG. 1 comes into a ratio of 1n:2n:4n:8n:16n:32n:64n:128n, wherein nmeans an integer determined by the sustaining pulse frequency.Accordingly, the brightness of the panel can be controlled within a greyscale between 0 and 256 in this case by selecting and combiningsubframes to lighten according to a grey scale to be displayed.

Since the combination of the sustaining pulses are provided usually in aROM table, it is possible to select one particular combination of thesustaining pulses in each subframe from the ROM table based on a desiredbrightness.

FIG. 3 is a schematic figure of a ROM table. In the example shown, foursubframes are provided and 128 combinations of sustaining pulses areprovided from addresses SUS0 to SUS127, in order to simplify theexplanation. Accordingly, by selecting a suitable ROM address based on adesired brightness, the number of sustaining pulses in the respectivesubframes is determined, thus allowing a panel display with the desiredbrightness.

For example, when ROM address SUS0 is selected in FIG. 3, the number ofsustaining pulses in subframe SF1 is one, that of in subframe SF2 istwo, that of in subframe SF3 four, and that of in subframe SF4 eight.Therefore, the total number of sustaining pulses contained in one frameis fifteen. On the other hand, when selecting ROM address SUS 127, SF1has 16 sustaining pulses, SF2 32 sustaining pulses, SF3 64 sustainingpulses and SF4 128 sustaining pulses, thus resulting in 240 sustainingpulses in total contained in one frame. Accordingly, a 16 timesdifference in brightness, that is, the ratio of 15 to 240, can beobtained.

In the respective subframes, the sustaining periods have differentlengths to each other while each reset period has the same length and sodo the address periods. In addition, as shown in FIG. 1, a quiescentperiod, in which no driving waveform is output, is provided after thelast subframe in each frame.

The above-mentioned driving method of the subframe mode is quitefundamental, and therefore, various changes are made to produce a realplasma display device. In the subframes shown in FIG. 1, for example,the number of sustaining pulses in each subframe is changed at aconstant ratio to obtain a constant grey scale display. However, it ispossible to set the number of sustaining pulses in high order subframes,for example, in subframes SF6, SF7 and SF8, to the same number as eachother in order to saturate the brightness. It is emphasized that theselection of the total number of sustaining pulses is not restricted toa constant ratio and different numbers among subframes may be used.

As explained above, a conventional plasma display device controls itsgrey scale display by selecting the number of sustaining pulses appliedduring a sustaining discharge period. On the other hand, the plasmadisplay device is connected with an external device, such as a TV tuner,a video tape deck and a personal computer to display image signals sentfrom such a device. In this case, various synchronizing signals are sentalong with the image signals from the external device. In general, thefrequency of the synchronizing signals differs depending on the kind ofexternal devices. Since the length of one frame in a display panel isdetermined based on the frequency of the synchronizing signals, aphenomenon arises that the frame length changes depending on a device towhich a plasma display device is connected.

This causes the following disadvantage. In a case where the frame lengthbecomes shorter than that assumed previously, one driving period of thePDP (reset period+address period+sustaining period, as shown in FIG. 1)exceeds the one frame length, thus resulting in an extraordinarydisplay.

The reset period and the address period are fixed to constant valuesrespectively which are set to be as short as possible. On the otherhand, the sustaining period varies depending on the number of sustainingpulses and the period of one sustaining pulse. In case where the ROMtable shown in FIG. 3 is used, the maximum values of the sustainingperiod in the respective subframes are 16T μsec. for subframe F1, 32Tμsec. for F2, 64T μsec. for F3 and 128T μsec. for F4, where T means aperiod of one sustaining pulse.

Accordingly, one driving period α in this case is given as follows:

[(one reset period+one address period)×number of subframes]+[16Tμsec.+32T μsec.+64T μsec.+128 μsec.].

One frame length which is required to conduct an ordinary display shouldexceed the value α (exactly, α+(one vertical fly-back period)). In casethat one driving period a exceeds one frame length, an ordinary displayis no longer conducted.

On the contrary, when one frame length becomes much shorter than thelength of one driving period due to the frequency change in asynchronizing signal externally input, the quiescent period becomesunnecessary longer, thus reducing the brightness.

As mentioned above, the conventional plasma display device driven in asubframe mode does not have enough applicability to various kind ofexternal devices which are driven based on different synchronizingsignals.

FIG. 4 shows the fundamental structure for realizing a function to copewith the frame length change in an external input signal according tothe present invention.

In this figure, 10 denotes a frame length calculating circuit forcalculating one frame length Tv based on one period of a verticalsynchronizing signal Vsync which is input from an external device, 11 asustaining pulse number detecting circuit for detecting the total numberof sustaining pulses contained in one frame according to brightnessinformation contained in an image signal from the external device, and12 a driving period length calculating circuit for calculating a realdriving period length Tg based on the total number of sustaining pulsesin one frame, the number of which is detected by the sustaining pulsenumber detecting circuit 11. The length of Tg of one driving period canbe calculated according to the following formula:

    [(one reset period+one addressing period)×number of subframes]+[total number of sustaining pulses×T]

wherein T means the pulse width of one sustaining pulse. As is clearfrom this formula, [(one reset period+one addressing period)×number ofsubframes] is a fixed values and the pulse width of a sustaining pulseis also fixed. As a result, the driving period length depends only onthe number of sustaining pulses.

In FIG. 4, 14 denotes comparing circuit to compare the calculated framelength Tv with the calculated length Tg of one driving period, andoutputs a compared signal S. 16 denotes changing circuit for changingthe number of total sustaining pulses on one frame or the number of scanlines. According to the first embodiment of the present inventionmentioned later, changing circuit 16 reduces the total number ofsustaining pulses contained in one frame in order to reduce the drivingperiod length Tg to less than the frame length Tv if comparing circuit14 finds Tv<Tg. Although this embodiment decreases the brightness of aPDP a little, an extraordinary display of the PDP is effectivelyavoided. When Tv>Tg, on the contrary, the total number of sustainingpulses in one frame is increased to increase the brightness of the PDP.

In another embodiment of the present invention, changing circuit 16decreases the number of display lines (scan lines) to shorten the lengthTg of one driving period until Tg becomes shorter than Tv. On thecontrary, when Tv>Tg, the number of display lines is increased. In ausual PDP, display cells are arranged in a matrix form, and each cell isscanned line sequentially. Therefore, the reduction of the number ofscan lines results irn the reduction of the length of an addressingperiod. For example, by interrupting the drive of some display lines,which are in upper and/or lower parts of the panel, to reduce the numberof display lines, respective addressing periods in each subframe areequally shortened, thus the length Tg of one driving period is reduced.As a result, Tg becomes shorter than Tv, to avoid an extraordinarydisplay on the PDP.

On the contrary, when comparing circuit 14 finds that Tv is larger thanTg, e.g., Tv>Tg, one can increase the number of display lines toincrease the length of an addressing period uniformly in each subframe.Thus, the number of display lines can be increased to maximize thenumber as long as Tv is larger than Tg.

In still another embodiment, it is possible that changing circuitchanges both the total number of sustaining pulses and the number ofdisplay lines to control the relationship between the frame length Tvand the length Tg of one driving period.

FIG. 5a is a block diagram showing the outline structure of a plasmadisplay device for realizing the respective embodiments of the presentinvention, and FIG. 6 shows a detailed structure of a part of thedisplay shown in FIG. 5a. In these figures, 20 denotes a PDP having aplurality of plasma cells 20a (light emitting cells) arranged in form ofa matrix, 21 an address driver, 22 a Y scan driver, 23 a Y commondriver, 24 an X common driver and 25 a control circuit for controllingthese drivers.

Control circuit 25 is comprised of a display data control part 26 and apanel drive control part 27. As shown in FIG. 6, the display datacontrol part 26 has a frame memory 26a, which temporally stores imagedata (DATA) from an external device and a data converter 26b, whichgives a predetermined signal processing and timing processing to thedata stored in frame memory 26a and outputs the resulting data toaddress driver 21. The panel drive control part 27 includes a scandriver control part 28 and a common driver control part 29 and generatesvarious timing signals based on vertical synchronizing signals Vsyncsupplied from the external device. The generated timing signals aresupplied to the display data control part 26, the Y scan driver 22, theY common driver 23 and the X common driver 24.

In one embodiment of the plasma display device according to the presentinvention, the common driver control part 29 contains the frame lengthcalculating circuit 10, the sustaining pulse number detecting circuit11, the driving period length calculating circuit 12, the comparingcircuit 14 and the changing circuit 16, as shown in FIG. 5b. Thesecircuits are usually formed with a microprocessor unit 29a and a memory29c in which a program for making the microprocessor unit 29a functionas these circuits is stored, as shown in FIG. 6.

It is further indicated in FIG. 5b that the common driver control part29 contains a time lapse detecting circuit 18 which is also realized bythe microprocessor unit and the program and whose function will beexplained later.

Address driver 21 generates address pulses using a high voltage supplyVa for selecting display cells and selectively applies these pulses toaddress electrodes of panel 20. Y scan driver 22 generates scan pulsesusing a high voltage supply Vs, which is for sustaining a display, andapplies these scan pulses line sequentially to Y electrodes of panel 20.These address pulses and scan pulses are generated during the addressingperiod in each subframe.

Y common driver 23 generates sustaining pulses using a high voltagesupply Vs for sustaining display, and applies these sustaining pulses toall the Y electrodes of panel 20 simultaneously. In a similar manner,the X common driver 24 generates sustaining pulses and a full-screenwriting pulse using the high voltage supply Vs for sustaining display.The full-screen writing pulses are applied to all the X electrodes ofpanel 20 simultaneously during the reset period of each subframe. Ofcourse, the sustaining pulses are applied to all the X electrodessimultaneously during the sustaining discharge period in each subframe.

FIG. 6 is a block diagram showing a part of the plasma display deviceshown in FIG. 5a, the part which is essential for realizing the functionshown in FIG. 4. As previously mentioned, the common driver control part29 includes microprocessor unit (referred to as MPU, below) 29a, a gatearray 29b and memory 29c in which a ROM table 29d for storingcombinations of sustaining pulses is further included. In FIG. 7, anexample of a ROM table 29d is shown for driving 8 subframes in oneframe. As shown in FIG. 6, scan driver control part 28 is comprised of ascan controller 28a.

The operation of the device shown in FIGS. 5 and 6 will be explainedbelow with respect to the function to realize the objects of the presentinvention.

Image data (display data, DATA) introduced from an external device, forexample, a TV, are stored first in frame memory 26a and converted intodigital data in data converter 26b, which digital data are then sent tothe common driver control part 29. MPU 29a in the common driver controlpart 29 calculates the actual length Tg of one driving period based onthe data from data converter 26b. In actuality, MPU 29a finds brightnessinformation from the data sent from data converter 26a and then findsthe total number of sustaining pulses in one frame by referring to ROMtable 29d which contains information regarding the number of subframesand the number of sustaining pulses in the respective subframes.

On the other hand, MPU 29a calculates one frame length Tv according tovertical synchronizing signals Vsync from the external device. Thecalculated values Tg and Tv are compared also in MPU 29c to determinethe correction value of the total number of sustaining pulses or thenumber of scan lines. Then, the correction value is sent to scancontroller 28a, which controls the driving period length Tg byincreasing or decreasing the number of scan lines or the address of ROMtable 29d.

Next, the various embodiment of the device shown in FIGS. 5 and 6 willbe explained with referring to flowcharts, each of which shows a programcondition in MPU 29a. Therefore, by changing a program in MPU 29c,various embodiments of the present invention can be realized.

In explaining the various embodiments, the ROM table shown in FIG. 7will be referred to. The ROM table shown is used in a PDP driven in asubframe mode and having an automatic power control function, fordetermining the upper limit value of the brightness (that is, the totalnumber of sustaining pulses). In a conventional PDP, when selecting, forexample, SUS 127 in the ROM table shown in FIG. 3, not only thebrightness but the power consumption becomes a maximum (in a case wherethe display ratio is equal to 100%). Since the display ratio is usuallyabout 30%, the power consumption of the PDP does not exceed a designedvalue even if SUS 127 is selected. However, when the display ratiounusually comes to about 100% or near to 100%, the power consumption mayexceed the designed value. Therefore, the automatic power controlfunction restricts the selection of addresses of the ROM table so as notto exceed a designed maximum value of brightness (referred to as MCBC,below). The ROM table shown in FIG. 7 indicates these maximum values ofbrightness. However, the ROM table of FIG. 7 is shown only as anexample, and therefore, the present invention is not restricted to theROM table used for such a special purpose.

The First Embodiment

The flowchart shown in FIG. 8 indicates processes to avoid anextraordinary display, which happens when one frame length derived froman input signal is shorter than one driving length of a PDP, bydecreasing the total number of sustaining pulses in one frame.

First, the length of one period of vertical synchronizing signal Vsyncis measured at step 100, and the measured value is set to be one framelength Tv for driving the PDP. Thereafter, at step 101, the length ofone driving period Tg is calculated as follows. First, the length of onereset period is added to that of one addressing period and the resultingvalue is multiplied by the number of subframes in one frame. In thiscase, the respective lengths of reset periods and addressing periods arefixed to the same values respectively over the entire subframes. Second,the total number of sustaining pulses is obtained from a ROM tableaddress which corresponds to the brightness of the input image signal.For example, when the brightness of the input image signal correspondsto address MCBC 126 shown in FIG. 7, the total number of sustainingpulses is obtained as 377 from the table. Based on this value, the totallength of sustaining discharge periods in one frame can be calculated,which is then added with the total length of the reset periods and theaddressing periods obtained as mentioned before, in order to derive thelength of one driving period Tg.

In the next step 102, the frame length Tv obtained at step 100 iscompared with the length Tg of one driving period obtained at step 101.When Tv is less than Tg, the difference Tr (=Tg-Tv) is obtained at step103. Thereafter, at step 104, Tr is divided by constant A, which isadequately determined in advance, to find a magnitude of reduction ofthe address value. The present address, for example, address MCBC 126,is thus decreased by the amount of Tr/A to obtain a corrected addressBCmax, for example, address MCBC 124.

By selecting an adequate value to be the constant A, the address of theROM table can be decreased sufficiently, and as a result, the totalnumber of sustaining pulses can be decreased sufficiently to shorten thelength Tg of one driving period to less than the one frame length Tv,thus avoiding an extraordinary display. For example, the addressreduction from MCBC 126 to MCBC 124 results in the reduction of thetotal number of sustaining pulses in one frame from 377 to 369, thusshortening the driving period Tg to less than the frame length Tv. Onthe other hand, when Tv is larger than Tg at step 102, the presentaddress MCBC is used without any address reduction.

If a relatively large value is selected as constant A, the address valueof the ROM table may not be reduced sufficiently to shorten the drivingperiod length Tg less than the frame length Tv. On the other hand, if arelatively small value is selected as constant A, such an inconveniencecan be avoided. In this case, however, a large address change occurs inthe ROM table, thus inviting an undesirable condition in which thebrightness change on the PDP is too great.

In order to avoid the above inconvenience, constant A may be selected tobe as large as possible and the output from step 104 may be connectednot to RETURN but to the input of step 101, as shown with a dotted linein FIG. 8. In other words, the calculation of the driving period lengthTg and the comparison between Tv and Tg are repeated by reducing anaddress value of the ROM table in short steps, thus making it possibleto detect an adequate address of the ROM table without inviting a largebrightness change.

Second Embodiment

The flowchart shown in FIG. 9 shows processes to increase the brightnessof a PDP by increasing the total number of sustaining pulses when adriving period length is shorter than a frame length determined by aninput synchronizing signal. In the embodiments shown below, the samereference numerals are adopted to the same or the similar steps, andtherefore, the explanation thereof will not be repeated in detail.

At step 100, one frame length Tv is obtained based on one period of aninput vertical synchronizing signal Vsync. Next, at step 101, onedriving period length Tg is obtained based on an address of the ROMtable, which address corresponds to the brightness of the input imagesignal. For example, when address MCBC 124 is used, it is found from theROM table that the total number of sustaining pulses in one frame is369. The driving period length Tg can be calculated based on the totalnumber of sustaining pulses thus obtained as mentioned in the firstembodiment.

Next, at step 200, the comparison between Tg and Tv is conducted to findif Tv>Tg. In case that Tv>Tg, i.e., the frame length determined by aninput synchronizing signal is longer than the driving period length, thecalculation Tv>Tg is conducted at step 201 to find the difference Tr. Atstep 202, Tr is divided by an adequately determined constant A, thusobtaining an increasing amount of address step, which is then added tothe present address, for example, MCBC 124, to obtain a correctedaddress BCmax, for example, MCBC 126. As a result, the brightness of thePDP is increased by an amount corresponding to the increase in the totalnumber of sustaining pulses, for example, from 369 to 377. In this case,as explained in the first embodiment, constant A may be set as large aspossible and the output of step 202 may be connected to the input ofstep 101 to repeat the processes from step 101 to step 202. As a result,a value of BCmax as large as possible can be obtained, thus allowing thebrightness of the PDP to be set to the highest value within a range inwhich an ordinary display is possible.

By combining the first embodiment and the second embodiment, anotherprocessing is possible as follows. In case that, as a result of theprocessing shown in FIG. 8, the frame length Tv becomes much longer thanthe driving period length Tg by lowering the ROM table address, forexample, from MCBC 126 to MCBC 122, then, the brightness of the PDP isincreased by conducting steps 200 to 202 shown in FIG. 9 to increase theROM table address, for example, up to MCBC 125. Thus, the brightness canbe increased to a maximum value as long as the ordinary display ispossible.

Third Embodiment

The first and the second embodiments assume that the frequency of aninput synchronizing signal does not change during the whole displayprocess. However, in a video tape recorder, for example, the frequency(60 Hz) in an ordinary playback mode is different from that (61.5 Hz) ofa quick playback mode. In addition, these modes are used repeatedly ingeneral. In such a case, when the driving mode is changed from theordinary playback mode to the quick playback mode, the frame length Tvbecomes shorter than before. Therefore, the total number of sustainingpulses should be reduced immediately according to the process shown inFIG. 8 to avoid an extraordinary display. However, as mentioned before,the quick playback mode and the ordinary playback mode are usedrepeatedly. Therefore, if the total number of sustaining pluses isincreased to raise the brightness of the PDP when the driving mode istemporarily set back from the quick playback mode to the ordinaryplayback mode, it should be decreased again after a short period toreduce the brightness at the next quick playback mode. This yields adisadvantage that the brightness of the PDP changes too frequently.

The present embodiment avoids such a disadvantage by not increasing thetotal number of sustaining pulses to increase the brightness of the PDPduring a temporally return from the quick playback mode to the ordinaryplayback mode, but by increasing the total number of sustaining pulsesto increase the brightness after the driving mode completely returns tothe ordinary playback mode.

To this end, the present embodiment adds the following steps to theflowchart shown in FIG. 8 as shown in FIG. 10: step 300 for resettingcounter CT to 0; step 301 for setting the value of counter CT to apredetermined value F after completing the comparison between Tv and Tg;step 302 for Judging the counter CT value to be 0 or not, aftercompleting the brightness correction in step 104; step 303 for reducingthe value in counter CT by one; and step 304 for resetting thebrightness value BCmax to the original value MCBC when the value ofcounter CT becomes 0 at step 302. In one embodiment of the presentinvention, these functions are realized by the time lapse detectingcircuit 18 in the common driver control part 29 shown in FIG. 5b.

According to the flowchart shown in FIG. 10, when the frame length of aninput signal changes, for example, from 60 Hz to 61.5 Hz due to anoperation change from the ordinary playback mode to the quick playbackmode, the total number of sustaining pulses is reduced immediately toconduct the ordinary display by implementing steps 100 to 103. In casethat the driving mode returns temporarily from the quick playback modeto the ordinary playback mode and Tv≧Tg at step 102, re-correction ofthe brightness is not conducted until counter CT counts F from 0. Inother words, the brightness is maintained at BCmax, which is of thebrightness during the quick playback mode. Once a predetermined time(determined by F) has passed while keeping the ordinary playback mode,this state is no longer deemed as a temporarily return from the quickplayback mode to the ordinary playback mode. Therefore, at step 304, thebrightness of the panel should be returned from BCmax to the originalbrightness MCBC, which is of the brightness for the ordinary playbackmode. Thus, a frequent change in brightness can be avoided even thoughthe ordinary playback mode and the quick playback mode are repeated.

The embodiments described above overcome the frame length change bychanging the total number of sustaining pulses. However, the embodimentsdescribed below overcome the frame length change by changing the numberof scan lines in the PDP. Changing the number of scan lines results inthe uniform change of the addressing periods in the respectivesubframes, for example, shown in FIG. 1. Accordingly, by reducing thenumber of scan lines, the length of one driving period becomes shorterwhile it becomes longer by increasing the number of scan lines.

Fourth Embodiment

In the embodiment shown in FIG. 11, Tv and Tg are obtained at steps 100and 101, Tv and Tg are compared in step 102, and the difference Trbetween Tv and Tg is derived in step 103 in the same manner as the firstembodiment which changes the total number of sustaining pulses toovercome the frame length change. Thereafter, at step 400, Tr/Tg1 issubtracted from NL, which is the number of scan lines at present, toderive a new number of scan lines NLmax, where Tg1 indicates a drivingperiod for scanning one display line. Thus, the respective addressingperiods can be reduced so that the length of one driving period becomesshorter than the one frame length to avoid an extraordinary display.

Fifth Embodiment

In the embodiment shown in FIG. 12, if Tv is found to be longer than Tgat step 200, then, the calculation of (Tv-Tg) is conducted at step 201to derive the difference Tr. Thereafter, at step 500, Tr is divided byTg1, which is of one driving period for one scan line, and the quotientTr/Tg1 is added to the present number of scan lines to derive acorrected number of scan lines NLmax. Thus, the number of scan lines isincreased to a maximum value as long as the condition Tv>Tg ismaintained.

Sixth Embodiment

The flowchart shown in FIG. 13 has an additional function, which is ableto avoid a frequent brightness change derived from a temporarilyfrequency changes of an input signal, in addition to the function of thefourth embodiment shown in FIG. 11, which overcomes the extraordinarydisplay problem by reducing the number of scan lines. This function isbased on the same necessity as that of the third embodiment, andtherefore, the explanation thereof will not be repeated here.

In this embodiment, new steps 600, 601, 602, 603 and 604 are added tothe flowchart of embodiment 4 shown in FIG. 11, wherein step 600 is toreset counter CT to 0, step 601 to set counter CT to be a predeterminedvalue F, step 602 to judge whether counter CT is 0 or not, step 603 todecrement counter CT by one, and step 604 to reset the corrected numberof scan lines NLmax, obtained by conducting steps 100 to 103 and 400, tothe original number of scan lines NL.

The detailed explanation of embodiment 6 is not described here becausethe processes of this embodiment are the same or similar to those of thethird embodiment except that the reduction of the total number ofsustaining pulses in the third embodiment is replaced by the reductionof the number of display lines.

The fourth to the sixth embodiments mentioned above are also effectiveas a supplemental technique for the existing countermeasures against amulti-scanning mode. For example, among PDPs driven in a subframe mode,there is proposed a PDP having a multi-Vsync function which is to reducethe number of subframes in synchronizing with the period of a verticalsynchronizing signal (Vsync). This function can, however, adjust thelength of a driving period using only the minimum subframe, for example,SF1, as one unit, thus resulting in a rough adjustment. On the contrary,a fine adjustment is possible by applying the present embodiment. Thisprovides a PDP having a high applicability to a wide range of frequency,in addition to the effect of reducing the number of the subframes.

Seventh Embodiment

In the first and the second embodiments mentioned above, the displaybrightness of a PDP changes according to the frequency change of aninput signal from an external device, to which the PDP is connected. Asa result, an event occurs in which one image is displayed with adifferent brightness when a PDP is connected to a different externaldevice, although the image essentially has the same brightness. Theflowchart shown in FIG. 14a shows the seventh embodiment of the presentinvention to cope with such an event.

In this embodiment, one frame length Tv is first calculated from oneperiod of an input synchronizing signal at step 100. Thereafter, at step700, the maximum number of sustaining pulses Nsus(Tv), which can beapplied during the one frame length Tv without causing an extraordinarydisplay, is calculated based on the frame length Tv obtained in step100. By multiplying Ysus, which is of a brightness of one sustainingpulse, by Nsus(Tv), the maximum brightness Ymax which can be displayedwithout causing any extraordinary display is obtained. On the otherhand, a brightness to be a reference value (predetermined brightness Yc)has been set in advance in order to fix the display brightness constantwithout depending on the frequencies of input signals.

At step 701, the obtained maximum brightness Ymax and the predeterminedbrightness Yc are compared. In a case where the values do not agree witheach other at step 701, the process moves to step 702 in whichcorrection of the present number of sustaining pulses is conducted tomake it agree with the predetermined brightness Yc. In actuality, thepredetermined brightness Yc is subtracted from the maximum brightnessYmax and the difference is divided by brightness Ysus for one sustainingpulse, thus obtaining a correction value of the total number ofsustaining pulses. Then, this correction value is subtracted from Nsus,which is the number of sustaining pulses capable of being displayed, andas a result, the number of sustaining pulses corresponding to thepredetermined brightness Yc is obtained.

FIG. 14b shows the structure of the common driver control part 29 shownin FIG. 5a, which structure is designed especially to realize theseventh embodiment. In fact, common driver control part 29 includes theframe length calculating circuit 10 for conducting the step 100, amaximum number of sustaining pulse and maximum brightness calculatingcircuit 291 for conducting the step 700, an agreement detecting circuit292 for conducting the step 701, and a maximum number of sustainingpulse setting circuit 293 for conducting the step 702 shown in FIG. 14a.These circuits 10, 291, 292, 293 and 294 are, of cause, structured withthe MPU 29a and the memory 29c shown in FIG. 6.

Accordingly, the present invention is capable of improving the displayquality of a PDP by so adjusting the total number of sustaining pulsesthat an image is displayed with the same brightness regardless that whatkind of external device is connected to the PDP.

As described above with reference to the preferred embodiments, theplasma display device according to the present invention is capable ofconducting an ordinary display even if it is connected to an externaldevice driven by a different frequency. This is because the plasmadisplay device can adjust the total number of sustaining pulses or thenumber of scanning lines so that the relationship between the framelength and the driving period length becomes proper. Thus, the presentinvention provides a plasma display device having a high applicabilityto a variety of external devices.

What is claimed is:
 1. A plasma display device comprising:a plasmadisplay panel; and a driving part for driving the plasma display panelin a subframe mode in which one frame for display is divided into aplurality of subframes and a predetermined number of sustaining pulsesis applied to the plasma display panel during each of the subframes soas to cause a sustaining discharge; said driving part furthercomprising,a first circuit for calculating a length of one frame fordisplay according to one period of a vertical synchronizing signal whichis input from an external device along with an image signal, a secondcircuit for detecting a total number of sustaining pulses contained inone frame based on a brightness information contained in said imagesignal, a third circuit for calculating a length of one driving periodof the plasma display panel necessary for displaying one frame based onthe total number of sustaining pulses detected by said second circuit, afourth circuit for comparing respective results from said first circuitand said third circuit, and a fifth circuit for changing a total numberof sustaining pulses in one frame according to a compared result in saidfourth circuit.
 2. The plasma display device according to claim 1,wherein said fifth circuit changes the total number of sustaining pulsesin a manner to reduce the same when said fourth circuit finds that thelength of one frame obtained by the first circuit is shorter than thelength of one driving period obtained by the third circuit.
 3. Theplasma display device according to claim 1, wherein said fifth circuitchanges the total number of sustaining pluses in a manner to increasethe same when said fourth circuit finds that the length of one frameobtained by the first circuit is longer than the length of one drivingperiod obtained by the third circuit.
 4. The plasma display deviceaccording to claim 1, wherein said driving part is further comprised ofa sixth circuit for detecting a lapse of a predetermined time period inwhich no change is detected once said fourth circuit has detected achange in a comparing result, and wherein said fifth circuit changes thetotal number of sustaining pulses after said sixth circuit detects thelapse of the predetermined time period.
 5. The plasma display deviceaccording to claim 1, wherein said first circuit, said third circuit andsaid fourth circuit are comprised of a microprocessor unit and arecording media in which a program for making said microprocessor unitfunction as said first, third and fourth circuit is stored.
 6. A plasmadisplay device comprising:a plasma display panel; and a driving part fordriving the plasma display panel in a subframe mode in which one framefor display is divided into a plurality of subframes and a predeterminednumber of sustaining pulses is applied to the plasma display panelduring each of the subframes so as to cause a sustaining discharge; saiddriving part further comprising,a ROM table having a plurality ofaddresses in which a combination of numbers of sustaining pulses forrespective subframes is stored, a first circuit for calculating a lengthof one frame for display according to one period of a verticalsynchronizing signal which is introduced from an external device alongwith an image signal, a second circuit for detecting a total number ofsustaining pulses contained in one frame based on an address of said ROMtable, said address which corresponds to brightness informationcontained in said image signal; a third circuit for calculating a lengthof one driving period of the plasma display panel necessary fordisplaying one frame based on the total number of sustaining pulsesdetected by said second circuit, a fourth circuit for comparing resultsfrom said first circuit and said third circuit; and a fifth circuit forchanging an address of the ROM table according to a compared result insaid fourth circuit.
 7. The plasma display device according to claim 6,wherein said fifth circuit changes an address of the ROM table in amanner to reduce a total number of sustaining pulses contained in theaddress when said fourth circuit finds that the length of one frameobtained by the first circuit is shorter than the length of one drivingperiod obtained by the third circuit.
 8. The plasma display deviceaccording to claim 6, wherein said fifth circuit changes an address ofthe ROM table in a manner to increase a total number of sustainingpulses contained in the address when said fourth circuit finds that thelength of one frame obtained by the first circuit is longer than thelength of one driving period obtained by the third circuit.
 9. Theplasma display device according to claim 6, wherein said driving part isfurther comprised of a sixth circuit for detecting a lapse of apredetermined time period during which no change is detected once saidfourth circuit has detected a change in a compared result, and whereinsaid fifth circuit changes the total number of sustaining pulses aftersaid sixth circuit detects the lapse of the predetermined time period.10. The plasma display device according to claim 1, wherein said firstcircuit, said third circuit and said fourth circuit are comprised of amicroprocessor unit and a recording media in which a program for makingsaid microprocessor unit function as said first, third and fourthcircuits is stored.
 11. A plasma display device comprising:a plasmadisplay panel having a plurality of light emitting cells arranged inform of a matrix; and a driving part for driving the plasma displaypanel in a subframe mode in which one frame for display is divided intoa plurality of subframes and a predetermined number of sustaining pulsesis applied to said plurality of light emitting cells in each subframewhile scanning the plurality of light emitting cells line-sequentially;said driving part further comprising,a first circuit for calculating alength of one frame for the display according to one period of avertical synchronizing signal which is introduced from an externaldevice with an image signal, a second circuit for detecting a totalnumber of sustaining pulses contained in one frame based on brightnessinformation contained in said image signal, a third circuit forcalculating a length of one driving period of the plasma display panelnecessary for displaying one frame based on the total number ofsustaining pulses detected by said second circuit, a fourth circuit forcomparing results from said first circuit and said third circuit; and afifth circuit for changing a total number of scan lines, which arescanned in line-sequence, according to a result from said comparingcircuit.
 12. The plasma display device according to claim 11, whereinsaid fifth circuit changes the total number of scan lines in a manner toreduce the same when said fourth circuit finds that the length of oneframe obtained by the first circuit is shorter than the length of onedriving period obtained by the third circuit.
 13. The plasma displaydevice according to claim 11, wherein said fifth circuit changes thetotal number of scan lines in a manner to increase the same when saidfourth circuit finds that the length of one frame obtained by the firstcircuit is longer than the length of one driving period obtained by thethird circuit.
 14. The plasma display device according to claim 11,wherein said driving circuit is further comprised of a sixth circuit fordetecting a lapse of a predetermined time period during which no changeis detected once said fourth circuit has detected a change in a comparedresult, and wherein said fifth circuit changes the total number of scanlines after said sixth circuit detects the lapse of the predeterminedtime period.
 15. The plasma display device according to claim 11,wherein said first circuit, said third circuit and said fourth circuitare comprised of a microprocessor unit and a recording media in which aprogram for making said microprocessor unit function as said first,third and fourth circuits is stored.
 16. A plasma display devicecomprising:a plasma display panel; and a driving part for driving theplasma display panel in a subframe mode in which one frame for displayis divided into a plurality of subframes and a predetermined number ofsustaining pulses is applied to the plasma display panel during each ofthe subframes so as to cause a sustaining discharge; said driving partfurther comprising,a first circuit for calculating a length of one framefor display according to one period of a vertical synchronizing signalwhich is introduced from an external device along with an image signal,a second circuit for calculating a maximum number of sustain pulses tobe displayed based on the length of one frame calculated by the firstcircuit and for calculating a maximum brightness according to themaximum number of sustaining pulses thus obtained; a third circuit fordetecting an agreement of the maximum brightness calculated by saidsecond circuit with a referential brightness determined in advance; anda fourth circuit for calculating a number of sustaining pulses whichcorresponds to said referential brightness when no agreement is detectedby said third circuit, and for setting the calculated number ofsustaining pluses to be the maximum number of sustaining pluses.
 17. Amethod for driving a plasma display device having a plasma display paneland a driving part for driving the plasma display panel in a subframemode in which one frame for display is divided into a plurality ofsubframes and a predetermined number of sustaining pulses is applied tothe plasma display panel during each of the subframes so as to cause asustaining discharge, the driving method comprising the stepsof:calculating a length of one frame for display according to one periodof a vertical synchronizing signal which is input from an externaldevice along with an image signal; detecting a total number ofsustaining pulses contained in one frame based on a brightnessinformation contained in said image signal; calculating a length of onedriving period of the plasma display panel necessary for displaying oneframe based on the total number of sustaining pulses detected at thedetecting step; comparing the length of one frame with the length of onedriving period obtained in the respective calculating steps; andchanging a total number of sustaining pulses in one frame according to acompared result obtained in the comparing step.